Aluminum melting furnace refractory



Feb. 19, 1963 .1. DOLPH 3,078,173

ALUMINUM MELTING FURNACE REFRACTORY Filed Nov. 8, 1960 2 Sheets-Sheet 2Composition A Composition B This invention Commercial standard 70-75aluminum alloy held 3 days at 15ooF,

Unite This invention relates to aluminum melting furnaces, and moreparticularly to essentially non vitreous refractories for theconstruction of such furnaces.

In the aluminum industry furnaces are used for remelting aluminum or forholding it in the molten state upon receiving it from other meltingfurnaces or from the electrolytic cells in which it is produced. Forbrevity all such furnaces are referred to herein as melting furnaces.The molten aluminum in such furnaces may receive other metals, such asmagnesium, manganese, zinc and copper, as alloying additions.

In secondary melting operations for the recovery of aluminum metal fromscrap, fluxes or refining agents are commonly added to the moltenaluminum. Thus, there may be additions of mixtures of chlorides, such assodium and potassium chlorides, aluminum and zinc chlorides, andadditions of cryolite, or chlorine gas may be bubbled through the metal.

The hearth and lower side walls which are in contact with the moltenaluminum are subject not only to erosion but also to corrosive attack bythe metal and by fluxes where they are used. Various types of refractorybrick have been tried for aluminum melting furnaces but the problem ofchemical attack has not been solved, particularly in the severestfiuxing practice and in the melting of corrosive aluminum alloys. Theattack on the refractory is especially severe at the metal line, wherethe level of the molten metal fluctuates during charging and tap ping,and due to fluxes and the buildup of dross. This [dross buildup, whichreduces furnace capacity, is composed principally of aluminum andaluminum oxide. Al-

though the dross can be removed with only moderate difficulty while itis still soft, it soon becomes very hard and so firmly attached to therefractory that its removal by mechanical means damages therefractories.

Another problem which must be considered is the fact that aluminumreacts with siliceous compounds, including silica, in the refractorieswith resultant contamination of the metal with silicon. Accordingly,when high purity metal is involved, refractories containing asubstantial percentage of available silicon compounds cannot betolerated.

For many years dense fire clay brick, such as those employed in blastfurnaces, were found to be satisfactory and, even today, continue togive acceptable service when there is little or no use of alloying orfluxing additions and silicon pick-up is not objectionable. However, thedemand for higher production rates, the present rigid alloyspecifications, and the corrosiveness of fluxes and alloying additionshave necessitated the development of new refractories to meet theserequirements.

Aluminum melts at about 1220 F., tapping temperatures exceed 1300 F.,and the metal temperature in the furnace ranges between 1300 and 1500 F.Temperatures of the furnace atmosphere are higher, of course, sincethese control the rate of working, and usually are 2000 F. or more. Thusthe refractories for these purposes must possess an adequate degree ofrefractoriness.

The most common mechanism of refractory attack in furnaces of the typeinvolved is marked by the penetration of the brick by the extremelyfluid molten aluminum metal, accompanied by such reactions as oxidationof the absorbed aluminum and the reduction of silicates to silicon, withdeposition of elemental silicon within the brick.

Patented Feb. 19, 1963 ice These reactions frequently cause expansion ofthe refractory and an inward heaving of the Wall, particularly at themetal line, as well as tending to cause spal'ling. Therefore theabsorption of aluminum or aluminum alloys by the refractory is indeedobjectionable. These brick must, moreover, be adequately strong andtough to withstand mechanical abuse from impact and abrasion caused bythe charging of ingots and scrap.

It is among the objects of this invention to provide essentiallynon-vitrified refractories of improved resistance to penetration andattack by molten aluminum, which possess strength and related propertiesrequisite to sustain the mechanical abuse just referred to, which are ofsimple composition, which may be made easily in accordance with normalrefractory production practices from well known and readily availablematerials, and which are comparable in cost to the better grades ofrefractories now used in furnaces for holding molten aluminum.

A further object is to provide furnaces for melting aluminum and forholding molten aluminum which exhibit minimized attack by liquidaluminum and its alloys in comparison with such furnaces presently inuse.

Yet another object is to provide a method to minimize attack of highalumina refractories by molten aluminum which is simple, economical,readily practiced and effectively minimizes deterioration of therefractories in use.

Other objects will appear from the following specification.

The invention will be described in connection with the accompanyingphotographs in which:

FIG. 1 is a view of a portion of a brick, which has been the commercialstandard for aluminum melting, after it had been exposed to moltenaluminum alloy;

FIG. 2 is a similar view of a portion of a brick according to thisinvention after exposure the same as FIG. 1;

FIG. 3 is a view of brick of FIG. 1 after longer exposure;

FIG. 4 is a view of brick of FIG. 2 after exposure the same as FIG. 3;and

FIGIS is a view of a section through a hearth in which molten aluminumhas been held and of which the right 7 hand half was constructed of thebrick of FIGS. 1 and 3,

and the left hand half of brick in accordance with this invention.

High alumina refractory brick have been widely used for the constructionof aluminum melting furnaces, because of their good physical propertiesand because their low silica content results in a minimum of siliconcontamination of the metal. However, all of the brick of this type knownheretofore have been subject, as far as I am aware, to penetration andchemical attack after a relatively short period of use. There has, as aresult, been a demand not only for a refractory possessing the physicalqualities of the high alumina type but alsopossessing greatly improvedresistance to the objectionable action of molten aluminum and itsalloys. v

I have discovered, and it is upon this that theinvention is primarilypredicated, that the objects of the invention are attained withrefractory compositions which contain, by weight, at least 50 percent ofA1 0 by analysis, supplied by refractory material of the group aluminaand high alumina ores, and which contain also a small percentage ofatleast one alkaline earth oxide, by analysis. The alkaline earth oxidemay be present in an amount from about 1 to 30 percent by weight,preferably about 2 to 8 percent, and it acts in some manner that is notwholly explicable to greatly increase the resistance of the refractoryto penetration of and attack by molten aluminum and alloying additions.Instead of alkal'ne earth oxide there may be used alkaline earthcompounds, such as the carbonates (e.g. dolomite or magnesite), thatprovide the stated amount of oxide. The term ,by analysis as used hereinmeans that upon analysis the stated amount will be found to be presenteven though combined with or in admixture with other substances.

For refractories of very high A1 content alumina will, of course, beused, suitably in the form known as tabular alumina. Where such high AlO content is not needed the A1 0 may be provided by bauxites or otheraluminum ores of high A1 0 content (about 50 percent or more). Ofcourse, such ores may be blended with alumina to supply an increasedcontent of A1 0 Thus the refractory will comprise alumina and thealkaline earth oxide together with constituents normal to the materialssupplying them. The compositions should be such that any glassy phase inthe fired product is, if present, in very low amount, i.e., the burnedproduct is essentially non-vitrified for any glassy phase is present inbut an insignificant amount. Also, because they exert a fluxing actionthat can seriously impair the properties of these refractories,especially their resistance to attack by aluminum, alkali metalcompounds should as far as possible be avoided but in any event shouldbe present in an amount less than 5 percent by weight of the refractory.

Alumina possesses little plasticity so that in the forming of shapedrefractories it is generally necessary to have present a plasticizingagent in order that the refractory shapes can be handled subsequent toforming. For many purposes ball clay is suitable but because of itssilicate nature it should be present in minor amount, say not in excessof percent, preferably not over 10 percent. Other non-mineralplasticizers can be used instead of clay, a variety of wh ch are knownand used in the re fractory trade, examples being lignin liquor,molasses, and carboxymethylcellulose. Such organic adjuvants are burnedout when the refractory is fired, or when the refractory is put in usein the case of chemically bonded and unfired shapes.

As will appear, other refractory materials, such as mag- Therefractories provided by this invention may be made by normal methods ofrefractory manufacture. Thus the raw materials may be crushed to abrick-making grind which will give a brick of high density, lowporosity, and good thermal shock resistance. Such grinds are quitestandard and they are well understood in the refractory trade. To obtainsuch properties the grind should be such as to provide a batch ofparticles graded from coarse to fine, but at least percent should becoarser than ISO-mesh (Tyler) because grinds containing much more than50 percent by weight finer than 150 mesh cannot be satisfactorilypressed into brick by standard brick-making practices. An example ofsuch a grind suited to the purpose of this invention is, by weight:

Percent 6 +10 mesh (Tyler) 15 -l0 +28 25 28 17 --65 43 The appropriategrind is thoroughly mixed in standard equipment and sutficient liquid,such as water or lignin liquor, is added to temper the batch, which isthen pressed or otherwise formed into brick under high pressure. Inusual practice the brick are then air dried followed by oven drying. Inthe case of chemically bonded brick, which are well understood in thetrade, the shapes are then ready for installation. Where fired brick areto be produced the dried brick are then fired at a temperature toproduce the desired physical properties but below a temperature thatwill cause vitrifaction.

The brick provided by the invention are useful for the construction ofthe hearths and side walls of aluminum melting furnaces, as will beclear from the tests reported hereinafter.

The following tables are exemplary of the practice of the invention andthe benefits to be derived from it:

Table I [Compositions in weight percent] ABE-IUDD-IEFF-IHJKL TabularAlumina 85 81 77 Ball Clay 15 15 15 Sinter A 4 B CaO (added as Ca.(OFI)0210 (added as CaCOa) CaO-l-MgO (added as Dolomite) BaO (added as B8003)B703 (added as B31301) So. Amer. Bauxite (calcined) Nevada Magnesitc(dead burned) Table II [The analysis of the materials was as follows]Tabular Alumina Ball Clay Siuter A Dolomite Cnlcined South NevadaAmerican Bauxite Mngnesite 99.4% 41,0; 30.3% A1103. 46.0% B10: 54.0%OaO. 88. 8% 4120:. 85.7% O. 0.3% S a. 53.6% SiOz. 23.8% 380 37.5% MgO.6.21% 810;. 5 09% CaO. 0.2% Fez03 1.7% T101. 17.7% Si0= 7.25%Fcroa-i-Alzoa. 3.37% 'IiOz. 4.34% SiOr. 0.1% Alkaline Earth 0.9% F920,.7.1% A120; 0.91% SiOz. 1.50% F8203. 4.05% F610;, Oxides. 0.49% CaO-l-MgO4.0% 030. 34% Ignition Loss. 0.79% A1201.

N370+K20+L120 0.5% Mg Alkahes. 12.4% Ignition Loss. 0.2% F8203.

0.1% TiOz.

nesite (an alkaline earth oxide) may be present in substantial amountsin these refractories with, commonly, a plasticizer of clay or bentoniteor of organic type. In either instance the refractory will, in use,consist essentially of alumina, alkaline earth oxide and refractorymaterial other than alumina, apart from oxides present in minor amounts,usually as impurities in the base materials. Commonly such other oxideswill be silica, t'tania, and iron oxide. Such other refractory materialshould be of low silica content.

In each example, the ingredients were crushed to the foregoing grind andthoroughly mixed dry. About 3 to 6 percent by weight of water was addedand the batches were then pressed into 9 x 4 /2 x 2 /2" straights atabout 4000 p.s.i. The shapes were air dried for about 24 hours and thenoven dried at about 230 F. overnight. Following this they were fired for10 hours at about 2550 F.

The effect of molten aluminum was determined by immersing specimens ofthe bricks about 2 x 2 x 2 inches to about one-half their height inmolten aluminum alloy 5 for 72 hours at 1500 F. In this test there wasused molten aluminum alloy 7075 which contains 5.5% of zinc, 2.5% ofmagnesium and 1.5% of copper. This alloy has been observed to attack thestandard high alumina brick very severely. The specimens were thenremoved on it and the temperature was maintained for three days. Aftercooling the hearth was sectioned vertically as shown in the photograph.Here again the extensive penetration of mix A with silicon is evidentwhile in striking contrast the bricks of mix B have not been penetrated/Examfrom the bath and cut in half to observe the penetration 5 inationof the photograph indicates slight dross accumucaused by the metal. Thistest simulates attack at the lation on the mix B bricks of thisinvention in contrast metal line where the attack is most severe. Theresults to appreciable accumulation on mixA bricks. of this test andothers when applied to the foregoing Shapes, such as bricks, may be madein various ways compositions are given in the following Table III. otherthan described above. For example, they may be Table III A B B-1 D D-l EWeight, p.c.f 171 170 167 162 162 156 166 Modulus of Rupture, p.s.i 1.050 2.630 3, 300 1, 370 1, 380 950 1,890 Apparent Porositv, percent 21.19.1 17. 8 25. 2 25. 2 28.1 23. 1 Linear Change in Burning, percent-+0.3 +0. 2 +0. 3 0.0 0.0 0.0 +0. 3 Aluminum Immersion Test, PenetrationRange, Inches to 1 0 to V0 1 0 to 8 0 0 to lie 0 0 to Me PrevailingPenetration 1" Max. 0 O 0 0 0 0 F F-l H J K L Weight, p.c.f 172 108 154160 165 146 Modulus of Rupture, p.s.i 1,280 920 1, 540 1.280 870 290Apparent Porosity, percent... 20. 7 20. 7 23.4 28 4 24.7 33.6 LinearChange in Burning, pe nt +0.2 +0. 5 +0.7 +3. 4 +1.7 +5.0 AluminumImmersion Test, Penetration Range, Inches 0 0 0 0 0 PrevailingPenetration 0 0 $4 0 0 0 1 Spot penetration.

It will be observed from Table III of test results that made by mixingfinely divided alkaline earth oxide and all of the bricks except Hshowed in comparison with the finely divided high alumina material, eg,bauxite, temperbase A brick a great improvement in resistance to pcneingthe mixture to an extruding consistency with water, tration by thealloy. Base mix A, which contained no extruding the mixture, drying andfiring the extruded alkaline earth addition, is a commercial brickheretofore dobie to about 2500 to 2700 F., thereby producing consideredto be the best for melting aluminum. Exama very dense dobie. Theresultant product is then ples B and B-1 were supplied with alkalineearth oxide crushed to a standard brick-making grind, such as that inthe form of sinter A, which as shown above contained given above, andbrick are made from it using standard in dditi n t 287% f C 0 d 13 0 a bt nti l protechniques. In this manner the alkaline earth oxide isportion of boric oxide. This led to investigating the efiect incorpo ineaCh grain Of the brick. of boric oxide in the form of-Example H but, asappears AS evidbncing the use Of large! amounts of alkaline from thetable, it was severely attacked. It may be noted earth Oxide than arerepresented y the compositionsiof also that much dross adhered toExamples A and H, a Table refefbnce may be m t Q test involving, littledross adhered to Examples B and B-1 while no dross y Weight, 704 PercentOf Crude a ha i a d dh d to h remaining spooimons 29.6 percent of barium carbonate. Grog and dobies Visual proof of the important characterofthe present W made i this mlxmre as i the lmme' invention is given bythe appended photographs which ihately precedmg paragraph and the fi Kburned represent the results of tests made in the manner and m the samey for hours The domes W h with the allo described above. Pro. 1 was abrick of ground and bmk were made mm the folbwmg 11X: composition A,Table 1. FIG. 2 shows the brick of Percent composition K, representativeof the present invention. 'Grog m m These specimens were immersed to adepth of 1 /2 inches crude South American bauxite 121 for three days inthe 7075 alloy at 1500 F. Two things n Clay 35 are immediately evidentupon examining these photo- Barium carbonate 50 graphs, namely, theextensive penetration of the com- Position A refractory, 1, Withdeposition f The brick were pressed and fired in the manner describedmental silicon and extensive dross accumulation on the i commotion i h Tbl I, A fi d h i h d 155 sides of the specimen whereas in contrast thecomposi- Pounds per bi foot h hd l f rupture was tion K refractory, FIG.2, showed no penetration what- 900 p s i their apparent porosity was 27percent, and SOBVeI and y a slight accumulation of dross On one theirlinear change in burning was minus 1.1 percent. fa FI 3 a 4 ShOW Whathad Occurred after These fired brick were then subjected to the aluminumthr W ks immersion in the Same alloy at the Same immersion testdescribed above in connection with the temperature. By this time thecommercial standard brick b i k f T bl 1 Th h d Zero penetration by ofcnmposition A had been Completely P 65 aluminum, both prevailing and asto range. There was trated and completely filled with elemental siliconand no dross b i1don them, d they showed l the Wi eXtfinSiVe drOSSacchmulatiOn- Yet Composition K faintest trace of reaction between thealuminum alloy (FIG. 4) after the same exposure showed only an d h frtory extremely narrow band of Penetration and insignificant Thecalculated chemical analysis of these latter brick is: drossaccumulation. 7 0

FIG. 5 is a section through a pilot hearth melting Percent furnace theright half of which was constructed from Alumina the commercial standardcomposition A while the left Silica half of which was constructed frommix B of Table I. a m Oxide 3 This was heated to 1500 F. and 7075 alloywas placed Remainder 2.7

The chemical analysis of the Alabama bauxite used in I this test was, onthe calcined basis:

Percent SiO 21.3 A1 75.0 Ti0 2.6 F5203 CaO 0.1 Ign. loss 0.1

Although the invention has been described with particular reference tothe production of shaped refractories, it may be applied in the form ofrefractory monoliths. Also, the refractories may contain otherrefractory materials in minor amounts that do not deleteriously affect 1the properties characteristic of the invention, such, for example, aszircon, chrome ore, zirconia, silicon carbide, and the like, due regardbeing had to the limitations set down above on alkali metal compounds,silica, and glasses phase.

This application is a continuation in part of my copending applicationSer. No. 822,190, filed June 23, 1959, now abandoned.

In accordance with the provisions of the patent statutes, I haveexplained the principle of my invention and have described what I nowconsider to represent its embodiment. However, I desire to have itunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

I claim:

1. A fired refractory brick, said brick characterized by resistance topenetration and reaction with molten aluminum and its alloys, andconsisting essentially of, by weight, from about 1 to 30% of a substanceof the group consisting of alkaline earth oxides and alkaline earthcompounds supplying that amount of alkaline earth oxide, the remaindersubstantially all material of the group consisting of alumina andaluminum ores containing, by oxide analysis, at least about 50% of A1 0said substance and said material being present in a brickmaking gradedsize range, at least about 50% thereof being coarser than 150 mesh, saidbrick being characterized as substantially free of any glassy phase andhaving less than about 30% apparent porosity.

2. That method of making refractory brick characterized by resistance topenetration by and reaction with molten aluminum and its alloys,comprising providing a batch of size graded refractory material at leastabout 50%, by weight, of which is coarser than 150 mesh, said materialconsisting essentially of, by weight, about 1m 30% of a substance of thegroup consisting of alkaline earth oxide and alkaline earth compoundssupplying that amount of alkaline earth oxide, the remainder of thebatch being substantially all material of the group consisting ofalumina and aluminum ores containing at least about A1 0 by weight andon the basis of an oxide analysis, tempering said batch, shaping thetempered batch to provide shapes having low porosity, drying the shapes,firing the shapes to a temperature below which any substantial glassyphase is produced.

3. A fired refractory brick, said brick characterized by resistance topenetration and reaction with molten aluminum and its alloys andconsisting essentially of, by weight, about 1 to 30% of a substance ofthe group consisting of alkaline earth oxides and alkaline earthcompounds supplying that amount of alkaline earth oxide, the remainderbeing substantially all high alumina bauxite, said substance and saidbauxite present in a brickmaking graded size range, at least about 50%thereof being coarser than 150 mesh, said brick being characterized assubstantially :free of any glassy phase, and said brick having less thanabout 30% apparent porosity.

4. A fired refractory brick, said brick characterized by resistance topenetration and reaction with molten aluminum and its alloys andconsisting essentially of, by

weight, about 1 to 30% of a substance of the group consisting ofalkaline earth oxides and alkaline earth compounds supplying that amountof alkaline earth oxide, about to of alumina, and the remaindersubstantially all ball clay, said substance, alumina, and ball claybeing present in a brickmaking graded size range, at least .about 50% ofwhich is coarser than mesh, and brick being characterized assubstantially free of any glassy phase, and said brick having less thanabout 30% apparent porosity.

5. The brick of claim 4 in which the substance is dead burned magncsite.

6. Brick according to claim 1 containing not over about 5 percent byweight of alkali metal oxides by analysis.

7. A method according to claim 2, the batch containing not over about 5percent by weight of alkali metal oxides by analysis.

8. Brick according to claim 4 containing not over about 5 percent byweight of alkali metal oxides by analysis.

References Cited in the file of this patent UNITED STATES PATENTS2,308,115 Schwartzwalder et al. Jan. 12, 1943 2,310,953 Heany Feb. 16,1943 2,760,875 Schwartzwalder et al. Aug. 28, 1956 FOREIGN PATENTS487,957 Great Britain June 29, 1938

1. A FIRED REFRACTORY BRICK, SAID BRICK CHARACTERIZED BY RESISTANCE TOPENETRATION AND REACTION WITH MOLTEN ALUMINUM AND ITS ALLOYS, ANDCONSISTING ESSENTIALLY OF, BY WEIGHT, FROM ABOUT 1 TO 30% OF A SUBSTANCEOF THE GROUP CONSISTING OF ALKALINE EARTH OXIDES AND ALKALINE EARTHOXIDE, POUNDS SUPPLYING THAT AMOUNT OF ALKALINE EARTH OXIDE, THEREMAINDER SUBSTANTIALLY ALL MATERIAL OF THE GROUP CONSISTING OF ALUMINAAND ALUMINUM ORES CONTAINING, BY OXIDE ANALYSIS, AT LEAST ABOUT 50% OFAL2O3, SAID SUBSTANCE AND SAID MATERIAL BEING PRESENT IN A BRICKMAKINGGRADED SIZE RANGE, AT LEAST ABOUT 50% THEREOF BEING COARSER THAN 150MESH, SAID BRICK BEING CHARACTERIZED AS SUBSTANTIALLY FREE OF ANY GLASSYPHASE AND HAVING LESS THAN ABOUT 30% APPARENT POROSITY.